1. Field of the Invention
The invention relates to an electrode utilized in electrochemical microcapacitor, and more particularly to an electrode with a one-dimensional conductive hybrid nanostructure.
2. Description of the Related Art
With the rapid development of microelectronic technology and biological engineering techniques, demand for various energy storage devices has increased. Electrochemical capacitors are popular because they offer higher energy density than conventional capacitors and higher power density than secondary cells and fuel cells. Although electrochemical capacitors (ultracapacitors) are similar to cells and conventional capacitors in structures, they possess advanced properties, for example, higher discharge power density and a longer charge/discharge cycle life than secondary cells. Additionally, electrochemical capacitors offer an energy density several thousand times greater than conventional capacitors.
Ruthenium dioxide crystals of rutile structure can serve as a low resistance (˜35 μΩcm) metal conductor as well as an electrochemical capacitor charge storage material. The charge of ruthenium on electrode surface is reduced from +4 to a lower state through series of redox reactions to bring into its unique property of pseudocapacitance. Although one-dimensional anhydrous ruthenium dioxide nanorods (RuO2NR) possess high conductivity that facilitates rapid charge/discharge, its comparatively low electrochemical area yields a mediocre capacitance thereof. Hydrous ruthenium dioxide is a mixed conductor of electron and proton and its nanosized rutile clusters intimately interfaced with water render itself proton conductive, resulting in high electrochemical area and capacitance. Hydrous ruthenium dioxide, however; exhibits lower electrical conductivity than anhydrous ruthenium dioxide.
Recently, research results regarding preparation of ruthenium dioxide capacitors have been reported, utilizing, for example, the laser direct-write technique to fabricate a hydrous ruthenium dioxide microcapacitor with high capacitance (see C. B. Arnold, J. Electrochem. Soc., 150, A571, 2003). The laser direct-write technique is, however, hardly a technique for mass production. An electrochemical capacitor comprising carbon nanotubes or titanium dioxide nanorods supporting electric double-layer capacitors and hydrous ruthenium dioxide serving as a charge storage material dispersed therein is disclosed (see Y. G. Wang, Electrochimica Acta, 49, 1957, 2004 and Y. T. Kim, J. Mater. Chem., 15, 4914, 2005). Such capacitors, however, has high capacitance and low power density. Additionally, an electrode structure comprising hydrous ruthenium dioxide or ruthenic acid serving as a charge storage material is disclosed (see W. Sugimoto, Chem. B, 109, 7330, 2005 and C. C. Hu, J. Electrochem. Soc., 151, A281, 2004). Such electrodes, however, exhibit low capacitance per unit area.
Thus, development of a ruthenium dioxide electrode with high capacitance and high power density is desirable.